CV% Is High

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Hi all! First time poster here. Recently ran GC and the data shows that our CV% was high. Just wanted to get an idea of why this would be... I'm new to running GC tests so I thought this would be a great place to learn the ropes. Thank you.
Welcome to the forum.

The first "rope" that you need to learn is that we can only help you if you tell us what you are doing and how. Knowing only that you are doing GC I could list several dozen reasons why you might be getting poor repeatability, most of which would be irrelevant to your circumstances.

So; what are you analysing, what concentrations, what is it dissolved in, how are you injecting, injection volume, inlet type, inlet liner, model of instrument, column type and dimension, gas flows and pressures, temperatures, what detector and detector settings, etc etc ?.

Peter
Peter Apps
Ok. I can only give you the basics since we ran it once, mostly unsupervised, and dont have access to it currently. We were testing antioxidant activity of oyster mushrooms in beef samples. We used 4-heptanone as the standard and we were looking for nonenal, pentanal, and octanal. I'll try and give you some details but unsure if they are what you need. This is all very new to me so my apologies.

-GC column (HP-1 fused silica capillary column)
-The GC tube was then attached to a purge and trap (OI analytical 4560) and purged for 15 min at 75°C.
-Our gas used was helium

Basically, I just need even one or two reasons why the CV may be high. It doesn't necessarily need to exactly apply to my situation. Just trying to get an idea of what could be a possible reason for error. Thank you.
Sadly the purge and trap just multiplies the possible sources of error.

Let's go at it from the other end. When you say high CVs is this in the results for the analytes (the aldehydes) or in the raw peak areas for the internal standard or the analytes ? Is the area for the internal std repeatable form run to run ?

If you only did it once, how can you calculate CV ? How many replicates (NB replicate analutical runs, not replicates of the addition of antioxidant to the beef) did you run or each sample and/or standard (presuming by "once" that you mean one batch of analyses) ? What was the CV, is it the same for each analyte ?

Peter
Peter Apps
Those Aldehydes are not particularly stable in water, especially when heated. The data I have for aldehydes in sw-846 method 8260 using 3050 (purge and trap) show poor reproducibility. That is why in EPA aldehyde methods the holding times are short, e.g. for 8315 it is just 3 days.
The solubility for those compounds in water would be low enough that you could reliably purge them at room temperature, or as low as 40C if you want to make sure you have consistent temperatures. The higher purge temperature will definitely cause more water to be sent to the trap which can cause multiple problems with chromatography.
The past is there to guide us into the future, not to dwell in.
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